1. Technical Field
The present invention relates to an apparatus for angle measurement, and to a method for angle measurement. An apparatus and a method according to the present invention for angle measurement can decisively improve the accuracy in the measurement of the angular position of shafts with large diameters, in both rotary tables and telescopic applications.
2. Background Information
High-precision angle measurement devices for measuring the angular position of a shaft, such as a shaft of a rotary table, are known. Variants that are self-supported and have a shaft on the end toward the measuring system are described in the book entitled “Digitale Längen-und Winkelmesstechnik” [Digital Length and Angle Measurement Technology] by A. Ernst, published by the Verlag Moderne Industrie, 3rd edition, 1998, pp. 61-64. To achieve high accuracy, first, highly accurate and hence very expensive precision bearings must be used in the measuring system. Second, connecting the measuring system shaft, which has a graduation plate with a radial measurement graduation, by a suitable coupling to the shaft that is to be measured requires major effort and expense.
Also, from pages 64-70 of the above mentioned book, angle measurement devices without self-support are known, in which a rotationally symmetrical measurement graduation or a corresponding graduation plate is disposed directly on a shaft that is to be measured. Corresponding scanning units for scanning the measurement graduation are disposed in stationary fashion relative to the rotating graduation plate.
In these angle measurement devices, incremental graduations with up to 36,000 radial lines are used as a measurement graduation, which further increases the angular resolution by interpolation.
In the ideal case, the accuracy of the angle measurement in such angle measurement devices depends on both the precision with which the measurement graduation was applied to the graduation plate, and on the measurement error of the scanning unit. In reality, due to manufacturing tolerances, the rotary motion of the graduation plate and, thus, of the measurement graduation, always has both an error in eccentricity and an error in wobble. This is due to the fact that: 1) the center point of the graduation plate can never lie exactly on the axis of rotation of the shaft to be measured, and 2) the axes of rotation of the graduation plate and of the shaft to be measured can never be disposed in exact alignment. As a consequence, the spacing and position of the measurement graduation relative to the scanning unit vary within one revolution of the shaft to be measured, resulting in a measurement error in the angle measurement.
To reduce the high expense for 1) precision bearings and precision couplings, especially in large angle measurement devices for shafts of large diameters, and 2) the mechanical calibration, German patent disclosure DE 199 07 326 A1 of the present Applicant proposes scanning the measurement graduation at a plurality of scanning points distributed over the circumference of the graduation plate, and evaluating the resultant sinusoidal signals and correcting errors in eccentricity and wobble.
Modern angle measurement devices, though, preferably use an absolutely coded code track instead of the incremental track. This code track is, for example, a multi-track code, such as a gray code, or a single-track incremental code, known as a “pseudo-random code” (PRC). This has the advantage that at every moment the absolute angle position can be determined directly by scanning the code track. However, the signal processing proposed in DE 199 07 326 A1 cannot be used for absolutely coded code tracks.